1. Technical Field
The present disclosure pertains to ultrasound imaging and, more particularly, to an ultrasound imaging system utilizing velocity vector estimation for generation of a vector Doppler color image in which a synthetic particle flow visualization method is employed.
2. Description of the Related Art
Ultrasound Imaging has developed into an effective tool for diagnosing a wide variety of disease states and conditions. The market for ultrasound equipment has seen steady growth over the years, fueled by improvements in image quality and the capability to differentiate various types of tissue. Unfortunately, there are still many applications for ultrasound systems where the equipment costs are too high for significant adoption. Examples are application areas such as breast cancer detection, prostate imaging, musculoskeletal imaging, and interventional radiology. In these areas and others, the diagnostic efficacy of ultrasound imaging depends on excellent spatial and contrast resolution for differentiation and identification of various tissue types. These performance capabilities are found only on the more expensive ultrasound systems, which have more extensive processing capabilities.
Ultrasound imaging has always required extensive signal and image processing methods, especially for array systems employing as many as 128 or more transducer elements, each with unique signal processing requirements. The last decade has seen a transition to the improved accuracy and flexibility of digital signal processing in almost all systems except for those at the lowest tiers of the market. This transition has the potential for reducing system costs in the long term by utilizing highly integrated digital circuitry. Unfortunately, the low manufacturing volumes of ultrasound systems results in substantial overhead and fixed costs for these unique circuits, and thus the transition to digital signal processing has not significantly reduced system cost.
Doppler methods in medical ultrasound encompass a number of related techniques for imaging and quantifying blood flow. For stationary targets, the round trip travel time of a pulse reflected from the target back to the transducer is the same for each transmission. Conversely, successive echographic returns from a moving object will arrive at different times with respect to the transmit pulse, and by cross correlating these echoes the velocity of the object can be estimated. Because the ultrasound path is directional (along the beam axis), only axial motion produces a Doppler signal. Flow that is transverse to the beam is not detectable, and thus the velocity magnitudes obtained in conventional Doppler methods represent only the axial component of the flow velocity vector. In order to estimate the true magnitude of the flow velocity vector, Vector Doppler methods are employed. Generally, these methods rely on multiple beam angle data to estimate the direction of the flow vector and the flow velocity vector.
Several Doppler-based methods have been developed to present different aspects of blood flow. Typically, “spatial imaging” of the flow field is used to locate vessels, to measure their size, and to observe flow structure. “Flow imaging” is used in conjunction with echographic imaging in a “duplex” mode that combines both types of images in an overlay, with echographic amplitude presented in grayscale and flow velocity rendered in color. The flow field is computed within a region of interest (ROI) that is a subset of the larger echographic image, because flow imaging is more demanding in both acquisition time and processing load.
Detailed quantification of flow velocity is possible within a much smaller sample volume chosen within the ROI. The smallest volume that can be sampled and processed independently is given by the axial length (the transmit pulse length) and the lateral beam widths (in and out of the imaging plane). Spatial resolution of any method depends on the size of the sample volume and also on the system sensitivity settings for that location.
The Spectral Doppler method reports the spectrum of flow velocity and how it varies over the cardiac cycle, and it usually presents the spectrum graphically as a spectrogram and audibly through loudspeakers. Moreover, the Spectral Doppler method computes the power spectrum of flow velocity obtained over a sequence of transmissions, and usually presents the spectrum graphically as a spectrogram and audibly through loudspeakers. Access to the full time-varying spectrum of blood velocities allows accurate calculation of mean and peak flow velocities within the sample region and provides the most complete characterization of flow disturbances of all the ultrasound Doppler methods.
Color Flow Doppler imaging of the velocity field within a region of interest is a method that presents flow using a color palette that typically renders higher velocities more brightly than slower ones, and distinguishes between different flow directions (generally toward the transducer or away from it) by using warm (reddish) and cool (bluish) tones. Very slowly moving and stationary regions are not colored, and a “wall filter” threshold is used to set the minimum cutoff velocity. Color Flow Doppler can provide approximate mean flow velocities in the region of interest, but accuracy is limited due to the short acquisition sequences needed to maintain reasonable frame rates.
Color Flow Doppler requires the acquisition of a rapid sequence of identical transmit-receive events, or “ensemble”, to detect and quantify motion by various means, essentially looking for correlated differences in arrival time, or phase, of the signal. The pulse repetition frequency (PRF) can be as fast as permitted by the round trip travel time of sound from the transducer to the maximum depth of the image and back again, but is generally adjusted to the minimum permitted to visualize peak blood velocities without aliasing. Typically, an ensemble of between 8 and 16 pulse-echo events is used for each Doppler scan line in the ROI. Choice of transmit beam focus parameters usually leads to Doppler scan lines that are 2 to 3 times broader than those used for echographic imaging. The requirement to transmit an ensemble of pulses in each beam direction generally leads to slower frame rates for Color Flow Doppler than for echographic imaging. Artifacts from slow frame rate can often be more noticeable in Doppler imaging than in grayscale echography because significant changes in flow can occur over a fraction of a cardiac cycle, and even slight probe motion may result in apparent flow over the entire ROI.
Using a small ROI can improve frame rates, but may limit the assessment of flow abnormalities. For example, a Color Flow ROI using 10 Doppler lines and ensembles of 12 pulses requires 120 events, similar to a full frame echographic image.
In general, high quality Doppler imaging is more technically difficult than echographic imaging in great part because backscattering from blood is very weak compared to tissue. Well known fundamental challenges to producing uncluttered and artifact-free Color Flow images include:                The requirement for highly repeatable transmit pulses, and very low noise and phase jitter in the acquisition hardware.        Flow signals are often of the same order of magnitude as various sources of noise, but averaging has an adverse impact on frame rate and other motion artifacts.        The large contrast between the scattering amplitudes of tissue and blood leads to difficulty in discriminating between vessel walls (strong echo) and moving blood (weak echo), even when the velocity contrast is high. In addition, blood flow velocity is often very slow near vessel walls, which often move (pulsate) in synchrony with the cardiac cycle.        Doppler pulses are typically longer than echographic pulses, and care must be taken to spatially register the flow and echo images which have different resolutions. This is particularly challenging for small blood vessels since the sample volume for Doppler pulses can be larger than the vessel diameter.        